18 February 2013

Like Marek Mauder in his blog post from 2009, I have run into a problem because of the different implementations of the right shift operator (shr in Pascal) in different languages.

There I was happily translating some code from Java to Object Pascal when I encountered Java's >>> operator. A quick look at the Java docs tells me this is a logical right shift. No problem, just use shr. Next I found Java's >> operator. This time its an arithmetic (sign preserving) right shift. Problem - no Pascal equivalent, and since operation was on signed integers I couldn't just use shr because there's a bug waiting to happen should any of the integers go negative.

Marek (above) said that replacing

a >> 1

with

adiv2;

worked for him.

This is fine for a single bit shift, so I thought I would see if it scaled for shifts of more than one bit. I tried dividing by 2 to the power of the number of bits to shift:

functionSAR(Value:LongInt;Shift:Byte):LongInt;

begin

Shift:=Shiftand31;

ifShift=0then

Exit(Value);

Result:=LongInt(LongWord(Value)shrShift);

ifValue<0then

Result:=LongInt(LongWord(Result)or($FFFFFFFFshl(32-Shift)));

end;

I can see several ways to optimise the function but all of them make the code less readable. So I'm not going to do the optimisations unless I have to.

I should be quite trivial to create 8, 16 and 64 bit integer overloads of the routine.